Performance Analysis for RSMA-Empowered STAR-RIS-Aided Downlink Communications

In order to support the need for higher spectral and energy efficiencies with a wider coverage area, simultaneous refracting/transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) and rate splitting multiple access (RSMA) have emerged as the potential technologies required for architectural advancement in the next-generation wireless communication networks. In this work, we propose a novel analytical framework of an RSMA-enhanced STAR-RIS-aided downlink multi-user communication system. First, we discuss the statistical characteristics of the different channels involved in the transmission and derive their probability density function (PDF). Using the derived PDF, we analyze the performance of the system and derive the analytical closed-form expressions of the outage probability at each reflecting and refracting downlink user for two different STAR-RIS operational protocols namely i) energy splitting (ES) and ii) mode switching (MS). Furthermore, we validate the accurateness of the all analytical expressions through Monte-Carlo (MC) simulations. We also highlight the impact of some important parameters of the system such as transmit power at the BS, elements in the STAR-RIS, imperfect channel state information (CSI) on the outage probability of each user. Finally, we demonstrate the dominance of RSMA over non-orthogonal multiple access (NOMA) on the system performance.